Speed up ungit

[jung-kim]

closes: #1091

• Speed optimizations at the graph component #1508
  • speed up logic for NodeViewModel.isViewable() logic by using a map instead of doing array.include()
  • throttling ko observable change triggers.
  • removed unnecessary observables.
  • cleaned up how refs are calculated for NodeViewModel. NodeViewModel.ref() is no longer depends on branchesAndLocalTags() and remoteTags().
  • introduced graph.isBigRepo() to disable some of the features
    • branches.js's branch sorting
    • animations
    • hover graphice ovrlay
  • graph.dropareaGraphActions to be computed

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Can someone test on their big repos?? It is an improvements but I'm not sure if it is enough of an improvement.

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Node count matters less because we incrementally load nodes. But the over all git ref count from show-ref -d command was the biggest reason for the slow down, and indirectly create nodes.

This PR was tested with a repo with 14498 refs and before ungit was unusable but now it usable without any big issues now.

[wmertens]

@jung-kim is there anything here that also works on the nodegit branch where node loading and graph layout is different?